Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)

On Fri, Apr 17, 2026 at 06:11:48PM +0100, Nick Alcock wrote:
> On 14 Apr 2026, Kris Van Hees uttered the following:
> 
> > Subject: Re: [PATCH 4/4] fbt: do not provide untraceable functins (for fprobes)
> 
> Typo.

Thanks.

> > Ensure that we do not provide FBT probes for functions that cannot be
> > probed using BPF fentry/fexit probes if fprobes are the default.
> 
> I guess falling back to kprobes for those single functions alone is
> either too complicated or has unpleasantly visible side-effects?

Someday, perhaps.  But right now the infrastructure cannot handle that.  The
implementation of probes for a provider is at the level of the provider and not
at the level of the probe.

> > Signed-off-by: Kris Van Hees <[email protected]>
> > ---
> >  libdtrace/dt_prov_fbt.c | 47 ++++++++++++++++++++++++++---------------
> >  1 file changed, 30 insertions(+), 17 deletions(-)
> >
> > diff --git a/libdtrace/dt_prov_fbt.c b/libdtrace/dt_prov_fbt.c
> > index 59e4583b2..a2f20c9bc 100644
> > --- a/libdtrace/dt_prov_fbt.c
> > +++ b/libdtrace/dt_prov_fbt.c
> > @@ -86,14 +86,30 @@ static int populate(dtrace_hdl_t *dtp)
> >  }
> >  
> >  /* Create a probe (if it does not exist yet). */
> > -static int provide_probe(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> > +static int provide_probe(dtrace_hdl_t *dtp, dt_module_t *dmp,
> > +			 dtrace_probedesc_t *pdp)
> >  {
> >  	dt_provider_t	*prv = dt_provider_lookup(dtp, pdp->prv);
> >  
> >  	if (prv == NULL)
> >  		return 0;
> > +	pdp->mod = dmp->dm_name;
> >  	if (dt_probe_lookup(dtp, pdp) != NULL)
> >  		return 0;
> > +
> > +	/*
> > +	 * The BPF verifier does not allow fentry/fexit probing of variadic
> > +	 * functions.
> 
> This comment is wrong now that this is being implemented via another
> function: the fact that dt_btf_func_is_traceable() checks variadicity
> belongs above that function (and is indeed there).  Something like

Oops, forgot to update that.

> /*
>  * Do not provide probes for functions with BTF that indicates that they
>  * are not traceable.
>  */
> 
> maybe?
> 
> > +	 */
> > +	if (prv->impl->prog_type == BPF_PROG_TYPE_TRACING) {
> > +		int32_t	btf_id;
> > +
> > +		btf_id = dt_btf_lookup_name_kind(dtp, dmp, pdp->fun, BTF_KIND_FUNC);
> > +		if (btf_id <= 0 ||
> > +		    !dt_btf_func_is_traceable(dtp, dmp->dm_btf, btf_id))
> > +			return -1;
> > +	}
> > +
> >  	if (dt_tp_probe_insert(dtp, prv, pdp->prv, pdp->mod, pdp->fun, pdp->prb))
> >  		return 1;
> >  
> 
> The hunks below appear to be doing something not called out in the
> commit log. I think this is getting the symbol module right?

This is necessary because the code above needs access to dmp.

> > @@ -173,16 +189,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >  
> >  			pd.id = DTRACE_IDNONE;
> >  			pd.prv = pdp->prv;
> > -			pd.mod = dmp->dm_name;
> >  			pd.fun = pdp->fun;
> >  
> >  			if (prb & FBT_ENTRY) {
> >  				pd.prb = "entry";
> > -				n += provide_probe(dtp, &pd);
> > +				n += provide_probe(dtp, dmp, &pd);
> >  			}
> >  			if (prb & FBT_RETURN) {
> >  				pd.prb = "return";
> > -				n += provide_probe(dtp, &pd);
> > +				n += provide_probe(dtp, dmp, &pd);
> >  			}
> >  
> >  			return n;
> > @@ -190,22 +205,20 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >  
> >  		sym = dt_symbol_by_name(dtp, pdp->fun);
> >  		while (sym != NULL) {
> > -			const char	*mod = dt_symbol_module(sym)->dm_name;
> > -
> > +			dmp = dt_symbol_module(sym);
> >  			if (dt_symbol_traceable(sym) &&
> > -			    dt_gmatch(mod, pdp->mod)) {
> > +			    dt_gmatch(dmp->dm_name, pdp->mod)) {
> >  				pd.id = DTRACE_IDNONE;
> >  				pd.prv = pdp->prv;
> > -				pd.mod = mod;
> >  				pd.fun = pdp->fun;
> >  
> >  				if (prb & FBT_ENTRY) {
> >  					pd.prb = "entry";
> > -					n += provide_probe(dtp, &pd);
> > +					n += provide_probe(dtp, dmp, &pd);
> >  				}
> >  				if (prb & FBT_RETURN) {
> >  					pd.prb = "return";
> > -					n += provide_probe(dtp, &pd);
> > +					n += provide_probe(dtp, dmp, &pd);
> >  				}
> >  
> >  			}
> > @@ -242,16 +255,15 @@ static int provide(dtrace_hdl_t *dtp, const dtrace_probedesc_t *pdp)
> >  
> >  		pd.id = DTRACE_IDNONE;
> >  		pd.prv = pdp->prv;
> > -		pd.mod = smp->dm_name;
> >  		pd.fun = fun;
> >  
> >  		if (prb & FBT_ENTRY) {
> >  			pd.prb = "entry";
> > -			n += provide_probe(dtp, &pd);
> > +			n += provide_probe(dtp, smp, &pd);
> >  		}
> >  		if (prb & FBT_RETURN) {
> >  			pd.prb = "return";
> > -			n += provide_probe(dtp, &pd);
> > +			n += provide_probe(dtp, smp, &pd);
> >  		}
> >  	}
> >  
> 
> The need for this trampoline change is also rather opaque.

Oops, that was meant to be in a different patch.  Will separate it out.

> > @@ -323,10 +335,11 @@ static int fprobe_trampoline(dt_pcb_t *pcb, uint_t exitlbl)
> >  		 */
> >  		dmp = dt_module_lookup_by_name(dtp, prp->desc->mod);
> >  		if (dmp && prp->argc == 2) {
> > -			int32_t	btf_id = dt_tp_probe_get_id(prp);
> > -			int	i = dt_btf_func_argc(dtp, dmp->dm_btf, btf_id);
> > -
> > -			emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_8, i * 8));
> > +			emit(dlp, BPF_MOV_REG(BPF_REG_1, BPF_REG_8));
> > +			emit(dlp, BPF_MOV_REG(BPF_REG_2, BPF_REG_FP));
> > +			emit(dlp, BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, DT_TRAMP_SP_SLOT(0)));
> > +			emit(dlp, BPF_CALL_HELPER(dtp->dt_bpfhelper[BPF_FUNC_get_func_ret]));
> > +			emit(dlp, BPF_LOAD(BPF_DW, BPF_REG_0, BPF_REG_FP, DT_TRAMP_SP_SLOT(0)));
> >  			emit(dlp, BPF_STORE(BPF_DW, BPF_REG_7, DMST_ARG(1), BPF_REG_0));
> >  		}
> >  	}
> 
> -- 
> NULL && (void)